1. Field of the Invention
The present invention relates to an acceleration sensor in which an acceleration detecting element and a liquid for giving damping to the acceleration detecting element are enclosed in a case.
2. Description of the Prior Art
Heretofore, an acceleration sensor for detecting the acceleration of a vehicle is known as shown in FIG. 6.
In FIG. 6, reference numeral 1 is a metallic case which is welded to a base 2. The case 1 is sealed airtightly. In the case 1, a ceramic board 3 is fixed to the base 2. A membrane resistance and the like are formed on the ceramic board 3. A semiconductor acceleration detecting element 5 for detecting acceleration is mounted on the ceramic board 3 with a silicon pedestal 4 therebetween. The semiconductor acceleration detecting element 5 is supported at only one end.
Further, in the case 1, silicon oil 6 is contained for giving damping to the acceleration detecting element 5, and a sponge 7 is mounted for absorbing the thermal expansion or contraction of the silicon oil 6 so as to avoid an accident such as leakage of the silicon oil 6. Reference numeral 8 is an adhesive substance with which the sponge 7 is caused to adhere to the case 1.
A strain gauge portion 5a is formed in the acceleration detecting element 5. A free end portion 5b of the strain gauge portion 5a is rocked in the direction of an arrow A, so that the strain gauge portion 5a is distorted. This distortion causes a change in a resistance value of the strain gauge portion 5a. Based on the change in the resistance value, the acceleration, which is applied in the direction of the arrow A, is detected.
Since the sponge 7 mounted in the conventional acceleration sensor is arranged so as to compensate for the thermal expansion or contraction of the silicon oil 6, there are instances where the sponge 7 cannot act according to the contraction of the silicon oil 6 in a low temperature. As a result, gas within the sponge 7 leaks and generates bubbles in the silicon oil 6. If the bubbles adhere to the surface of the free end portion 5b of the acceleration detecting element 5, friction characteristics are changed between the acceleration detecting element 5 and the silicon oil 6. Thereby, the amount of rocking of the free end portion 5b of the acceleration detecting element 5 is changed. Accordingly, the amount of distortion of the strain gauge portion 5a is changed and thereby the output characteristics of the detecting element 5 are fluctuated. Therefore, disadvantageously, an accurate acceleration cannot be detected.